Honeycomb filter and method for producing the honeycomb filter

ABSTRACT

A honeycomb filter includes an outer wall having a substantially cylindrical shape and a central axis of the substantially cylindrical shape, first walls provided at an inner side of the outer wall and extending along the central axis, and second walls provided at the inner side of the outer wall and extending along the central axis. The first walls and the second walls substantially perpendicularly cross to define cells having a substantially square shape in a cross-section perpendicular to the central axis. A thickness of the outer wall in the cross-section at a first intersection between the outer wall and a first diameter substantially parallel to the first walls in the cross-section and a thickness of the outer wall in the cross-section at a second intersection between the outer wall and a second diameter substantially parallel to the second walls in the cross-section are largest.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of Ser. No. 12/032,255filed Feb. 15, 2008, which in turn is a continuation application of Ser.No. 11/230,844 filed Sep. 21, 2005, which in turn is a continuationapplication of Ser. No. 10/671,418 filed Sep. 26, 2003, which is nowU.S. Pat. No. 7,112,233, patented Sep. 26, 2006; which in turn is acontinuation application of Ser. No. 09/856,751 filed Jul. 30, 2001,which is now U.S. Pat. No. 6,669,751, patented Dec. 30, 2003; which inturn is the U.S. National application of PCT application PCT/JP00/06599filed Sep. 26, 2000. The contents of these applications and patents areincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a honeycomb filter and a method forproducing a honeycomb filter.

2. Discussion of the Background

The number of automobiles has increased drastically this century. As aresult, the amount of gas discharged from automobile engines hascontinued to increase proportionally. Various substances suspended inthe exhaust gas that is emitted, especially from diesel engines, causepollution and severely affect the environment. Further, recentlyreported research results have shown that the fine particles suspendedin gas emissions (diesel particulates) may cause allergies or decreasesperm counts. Thus, actions to eliminate the fine particles suspended ingas emissions must immediately be taken for the sake of mankind.

Due to this situation, many exhaust gas purification apparatuses havebeen proposed in the prior art. A typical exhaust gas purificationapparatus includes a casing, which is located in an exhaust pipeconnected to an exhaust manifold of an engine, and a filter, which isarranged in the casing and has fine pores. In addition to a metal or analloy, the filter may be formed from ceramic. A cordierite honeycombfilter is a known example of a ceramic filter. Recent filters are oftenformed from sintered porous silicon carbide body that is advantageousfrom the viewpoints of heat resistance and mechanical strength, has ahigh accumulating efficiency, is chemically stable, and has a smallpressure loss.

The pressure loss refers to the difference between the pressure valuetaken upstream of the filter and the pressure value taken downstream ofthe filter. A main cause of power loss is the resistance the exhaust gasencounters when passing through a filter.

The honeycomb filter includes a plurality of cells extending along theaxial direction of the honeycomb filter. When the exhaust gas passesthrough the filter, the walls of the cells trap fine particles. Thisremoves fine particles from the exhaust gas.

However, the honeycomb filter, which is made of a sintered poroussilicon carbide body, is vulnerable to thermal impacts. Thus, largerfilters are liable to crack. Accordingly, a technique for manufacturinga large ceramic filter assembly by integrating a plurality of smallfilters has recently been proposed to prevent breakage resulting fromcracks.

A typical method for manufacturing a ceramic filter assembly will now bediscussed. First, ceramic raw material is continuously extruded from amold of an extruder to form an elongated square honeycomb moldedproduct. After the honeycomb filter is cut into pieces of equal length,the cut pieces are sintered to form a filter. Subsequent to thesintering process, a plurality of the filters are bundled and integratedby adhering the outer surfaces of the filters to each other with aceramic seal layer having a thickness of 4 to 5 mm. This completes thedesired ceramic filter assembly.

A mat-like thermal insulative material, made of ceramic fiber or thelike, is wrapped about the outer surface of the ceramic filter assembly.In this state, the assembly is arranged in a casing, which is located inan exhaust pipe.

However, in the prior art, there is a shortcoming in that the fineparticles trapped in the ceramic filter assembly do not burn completelyand some of the fine particles remain unburned. Accordingly, theefficiency for processing the exhaust gas is low.

Further, the honeycomb filter of the prior art has corners. Thus, thereis a tendency of stress concentrating on the corners of the outersurface and chipping the corners. Further, the seal layer may crack andbreak the ceramic filter assembly from the corners thereby damaging theentire ceramic filter assembly. Even if the assembly does not break,there is a shortcoming in that leakage of the exhaust gas may decreasethe processing efficiency.

During usage of the filter assembly, a high temperature differencebetween the honeycomb filters may cause thermal stress to crack thehoneycomb filters and break the entire assembly. Thus, the strength ofeach honeycomb filter must be increased to increase the strength of thehoneycomb filter assembly.

The prior art ceramic filter assembly as a whole has a rectangularcross-section. Thus, the periphery of the assembly is cut so that theassembly as a whole has a generally round or oval cross-section.

However, the filter has a plurality of cells. Thus, if the periphery ofthe assembly is cut, the cell walls are exposed from the peripheralsurface subsequent to the cutting. This forms lands and pits on theperipheral surface. Thus, even if the assembly is accommodated in thecasing with the thermal insulative material attached to the peripheralsurface of the assembly, gaps are formed in the longitudinal directionof the filters. Thus, exhaust gas tends to leak through the gaps. Thislowers the processing efficiency of the exhaust gas.

With regard to diesel particulates trapped in the honeycomb filter, ithas been confirmed that particulates having a small diameter have a highlung attaching rate and increase the risk to health. Thus, there isgreat need to trap small particulates.

However, when the pore diameter and the porosity of the honeycomb filterare small, the honeycomb filter becomes too dense and hinders smoothpassage of the exhaust gas, which, in turn, increases the pressure loss.This lowers the driving performance of the vehicle, lowers fuelefficiency, and deteriorates the driving performance.

On the other hand, if the pore diameter and porosity rate are increased,the above problems are solved. However, the number of openings in thehoneycomb filter becomes too large. Thus, fine particles cannot betrapped. This decreases the trapping efficiency. Further, the mechanicalstrength of the honeycomb filter becomes low.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a honeycomb filterincludes an outer wall, first walls, and second walls. The outer wallhas a substantially cylindrical shape and a central axis of thesubstantially cylindrical shape. The first walls are provided at aninner side of the outer wall and extending along the central axis. Thesecond walls are provided at the inner side of the outer wall and extendalong the central axis. The first walls and the second wallssubstantially perpendicularly cross to define cells having asubstantially square shape in a cross-section perpendicular to thecentral axis. A thickness of the outer wall in the cross-section at afirst intersection between the outer wall and a first diametersubstantially parallel to the first walls in the cross-section and athickness of the outer wall in the cross-section at a secondintersection between the outer wall and a second diameter substantiallyparallel to the second walls in the cross-section are largest.

According to another aspect of the present invention, a honeycomb filterincludes cell walls extending along a longitudinal direction of thehoneycomb filter to define cells between the cell walls. The cells havefirst cells and second cells each having a substantially square shape ina cross-section perpendicular to the longitudinal direction. The firstcells are sealed and the second cells are open at one end face of thehoneycomb filter. The substantially square shape of the first cellsinclude a first side whose extended line intersects a second side of thesubstantially square shape of the second cells. The second side and theextended line of the first side are substantially perpendicular.

According to further aspect of the present invention, a method forproducing a honeycomb filter includes providing a ceramic raw materialslurry. The ceramic raw material slurry is provided into an extruder toobtain a molded honeycomb filter. The molded honeycomb filter issintered. The molded honeycomb filter includes an outer wall, firstwalls, and second walls. The outer wall has a substantially cylindricalshape and a central axis of the substantially cylindrical shape. Thefirst walls are provided at an inner side of the outer wall andextending along the central axis. The second walls are provided at theinner side of the outer wall and extending along the central axis. Thefirst walls and the second walls substantially perpendicularly cross todefine cells having a substantially square shape in a cross-sectionperpendicular to the central axis. A thickness of the outer wall in thecross-section at a first intersection between the outer wall and a firstdiameter substantially parallel to the first walls in the cross-sectionand a thickness of the outer wall in the cross-section at a secondintersection between the outer wall and a second diameter substantiallyparallel to the second walls in the cross-section are largest.

According to further aspect of the present invention, a method forproducing a honeycomb filter includes mixing ceramic particles and abinder to obtain a ceramic raw material slurry. The ceramic raw materialslurry is provided into an extruder to obtain a molded honeycomb filter.The molded honeycomb filter includes cell walls extending along alongitudinal direction of the molded honeycomb filter to define cellshaving a substantially square shape. The molded honeycomb filter isdried using a microwave dryer. The dried molded honeycomb filter issintered.

According to the other aspect of the present invention, a method forproducing a honeycomb filter includes providing a ceramic raw materialslurry. The ceramic raw material slurry is provided into an extruder toobtain a molded honeycomb filter. The molded honeycomb filter issintered. The molded honeycomb filter includes cell walls extendingalong a longitudinal direction of the honeycomb filter to define cellsbetween the cell walls. The cells have first cells and second cells eachhaving a substantially square shape in a cross-section perpendicular tothe longitudinal direction. The first cells are sealed and the secondcells are open at one end face of the honeycomb filter. Thesubstantially square shape of the first cells include a first side whoseextended line intersects a second side of the substantially square shapeof the second cells. The second side and the extended line of the firstside are substantially perpendicular.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an exhaust gas purification apparatusaccording to a first embodiment of the present invention.

FIG. 2 is a perspective view showing a ceramic filter assembly of theexhaust gas purification apparatus of FIG. 1.

FIG. 3 is a perspective view showing a honeycomb filter of the ceramicfilter assembly of FIG. 2.

FIG. 4 is an enlarged cross-sectional view showing the main portion ofthe exhaust gas purification apparatus of FIG. 1.

FIG. 5 is an enlarged cross-sectional view showing the main portion ofthe ceramic filter assembly of FIG. 2.

FIG. 6 is an enlarged cross-sectional view showing the main portion of aceramic filter assembly of a first modified example.

FIG. 7 is a perspective view showing a honeycomb filter according to asecond embodiment of the present invention.

FIG. 8 is an enlarged cross-sectional view showing the main portion of aceramic filter assembly.

FIG. 9 is an enlarged cross-sectional view showing the main portion of aceramic filter assembly according to a first modified example.

FIG. 10 is a perspective view showing the honeycomb filter according tothe first modified example.

FIG. 11 is a perspective view showing a honeycomb filter according to asecond modified example.

FIG. 12 is a perspective view showing a honeycomb filter according to athird modified example.

FIG. 13 is a side view showing a ceramic filter assembly according to athird embodiment of the present invention.

FIGS. 14( a) to 14(c) are schematic perspective views illustrating amanufacturing process of the ceramic filter assembly of FIG. 13.

FIG. 15 is a side view showing a ceramic filter assembly according to amodified example.

FIG. 16 is a perspective view of a ceramic filter assembly according toa fourth embodiment of the present invention.

FIG. 17 is a perspective view showing a filter of the ceramic filterassembly 3 of FIG. 16.

FIG. 18( a) is a schematic cross-sectional view showing the filter ofFIG. 17, and FIG. 18( b) is a schematic side view showing the filter ofFIG. 17.

FIG. 19 is a perspective view showing a honeycomb filter provided with ahoneycomb structure according to fifth and sixth embodiments of thepresent invention.

FIG. 20 is a cross-sectional view showing the filter 59 of FIG. 19 takenalong line 20-20.

FIG. 21 is an enlarged cross-sectional view showing the main portion ofan exhaust gas purification apparatus.

FIG. 22 is a perspective view showing a ceramic filter assembly.

DESCRIPTION OF THE EMBODIMENTS

A diesel engine exhaust gas purification apparatus 1 according to afirst embodiment of the present invention will now be described withreference to FIGS. 1 to 5.

Referring to FIG. 1, the exhaust gas purification apparatus 1 is anapparatus for purifying the exhaust gas emitted from a diesel engine 2,which serves an internal combustion engine. The diesel engine 2 has aplurality of cylinders (not shown). Each cylinder is connected to abranch 4 of an exhaust manifold 3, which is made of a metal material.Each branch 4 is connected to a single manifold body 5. Accordingly, theexhaust gas emitted from each cylinder is concentrated at one location.

A first exhaust pipe 6 and a second exhaust pipe 7, which are made of ametal material, are arranged downstream to the exhaust manifold 3. Theupstream end of the first exhaust pipe 6 is connected to the manifoldbody 5. A tubular casing 8 made of a metal material is arranged betweenthe first exhaust pipe 6 and the second exhaust pipe 7. The upstream endof the casing 8 is connected to the downstream end of the first exhaustpipe 6, and the downstream end of the casing 8 is connected to theupstream end of the second exhaust pipe 7. With this structure, it maybe considered that the casing 8 is arranged in the exhaust pipes 6, 7.The first exhaust pipe 6, the casing 8, and the second exhaust pipe 7are communicated with each other so that exhaust gas flows therethrough.

As shown in FIG. 1, the middle portion of the casing 8 has a diameterlarger than that of the exhaust pipes 6, 7. Accordingly, the interior ofthe casing 8 is larger than that of the exhaust pipes 6, 7. A ceramicfilter assembly 9 is accommodated in the casing 8.

A thermal insulative material 10 is arranged between the outer surfaceof the assembly 9 and the inner surface of the casing 8. The thermalinsulative material 10 is a mat-like material including ceramic fibersand has a thickness of several millimeters to several tens ofmillimeters. It is preferred that the heat insulative material 10 bethermally expansive. Thermally expansive refers to the release ofthermal stress through an elastic structure. This is to minimize energyloss during reproduction by preventing heat from being released from theoutermost portion of the assembly 9. Further, the expansion of ceramicfibers using the heat produced during reproduction prevents displacementof the ceramic filter assembly 9, which would result from the pressureof the exhaust gas or vibrations produced by the moving vehicle.

The ceramic filter assembly 9 eliminates diesel particulates and it thusnormally referred to as a diesel particulate filter (DPF). As shown inFIG. 2 and FIG. 4, the assembly 9 is formed by bundling and integratinga plurality of filters F. Elongated square filters F1 are arranged atthe central portion of the assembly 9, and the outer dimension of theelongated square filter F1 is 33 mm×33 mm×167 mm (refer to FIG. 3).Filters F1 that have forms differing from the elongated square filtersF1 are arranged about the elongated square filters F1. This forms theceramic filter body 9, which as a whole, is cylindrical (diameter beingabout 135 mm).

These filters F1 are made of a sintered porous silicon carbide, which isone type of sintered ceramic. The reason for employing sintered poroussilicon carbide is because it is advantageous especially in that it hassuperior heat resistance and heat conductance. In addition to sinteredporous silicon carbide, the sintered material may be silicon nitride,sialon, alumina, cordierite, or mullite.

As shown in FIG. 3 and the other drawings, the filters F1 have ahoneycomb structure. The reason for employing the honeycomb structure isin that the pressure loss is small when the trapped amount of fineparticles increases. Each filter f1 has a plurality of through holes 12,which have generally square cross-sections and are arranged regularlyextending in the axial direction. The through holes 12 are partitionedfrom each other by thin cell walls 13. The outer surface of the cellwall 13 carries an oxide catalyst formed from a platinum group element(such as Pt) or other metal elements and there oxides. The opening ofeach through hole 12 on one of the end surfaces 9 a, 9 b is sealed by asealing body 14 (sintered porous silicon carbide body). Accordingly, theend surfaces 9 a, 9 b have a chessboard appearance. Thus, the filters F1have a plurality of cells having square cross-sections. The cell densityis about 200/inch, the thickness of the cell wall 13 is about 0.3 mm,and the cell pitch is about 1.8 mm. Among the plurality of cells, abouthalf are opened to the upstream end surface 9 a, and the others areopened at the downstream end surface 9 b.

The average porous diameter of the filter F1 is about 1 μm-50 μm, andmore particularly, 5 μm-20 μm. If the average pore diameter is less than1 μm, the deposited fine particles tend to clog the filter F1. If theaverage pore diameter exceeds 50 μm, fine particles would not be trappedand would decrease the trapping efficiency.

It is preferred that the porosity rate be 30% to 70%, and moreparticularly, 40% to 60%. If the porosity rate is lower than 30%, thefilter F1 becomes too fine and may hinder the circulation of exhaust gastherein. If the porosity rate exceeds 70%, the amount of gaps in thefilters F1 becomes too large. This may decrease the strength of thefilters f1 and decrease the fine particle trapping efficiency.

When selecting the sintered porous silicon carbide, it is preferred thatthe heat conductance of the filter F1 be 20 W/mK to 80 W/mK, and moreparticularly, 30 W/mK to 70 W/mK.

Referring to FIGS. 4 and 5, the outer surfaces of a total of 16 filtersF are adhered to one another by means of a ceramic seal layer 15.

The ceramic seal layer 15 will now be described in detail.

It is preferred that the heat conductance of the seal layer 15 be 0.1W/mK-10 W/mK, and more particularly be 0.2 W/mK-2 W/mK.

If the heat conductance is less than 0.1 W/mK, the heat conductance ofthe seal layer 15 cannot be sufficiently improved. Thus, the seal layer15 continues to be a large resistance and hinders heat conductionbetween filters F1. On the other hand, if the heat conductance exceeds10 W/mK, properties such as adhesion and heat resistance may be degradedand cause manufacturing to be difficult.

It is required that the thickness t1 of the seal layer 15 be 0.3 mm-3mm. Further, it is preferred that the thickness be 0.5 mm-2 mm.

If the thickness t1 exceeds 3 mm, the seal layer 15 continues to be alarge seal layer 15 even if the heat conductance is high and the heatconductance between the filters F1 is hindered. In addition, the ratioof the assembly 9 occupied by the filters F1 would relatively decreaseand lower the filtration capacity. On the other hand, if the thicknesst1 of the seal layer 15 is less than 0.3 mm, the seal layer 15 would notbecome a large resistance. However, the force adhering the filters F1 toeach other may become too low and cause the assembly 9 to be vulnerableto breakage.

The seal layer 15 is formed from at least an inorganic fiber, aninorganic binder, an organic binder, and inorganic particles. Further,it is preferred that the seal layer 15 be an elastic material formed bybinding inorganic fibers and inorganic particles, whichthree-dimensionally intersect one another, with an inorganic binder andan organic binder.

At least one type of ceramic fiber selected from silica-alumina fiber,mullite fiber, alumina fiber, and silica fiber are selected as theinorganic fiber included in the seal layer 15. Among these fibers, it ismost preferred that silica-alumina ceramic fiber be selected.Silica-alumina ceramic fiber has superior elasticity and serves toabsorb thermal stress.

In this case, the content of the silica-alumina ceramic fiber in theseal layer 15 is 10 wt %-70 wt %, preferably 10 wt %-40 wt %, and morepreferably 20 wt %-30 wt %. If the content is less than 10 wt %, thethermal conductivity decreases and the elasticity decreases. If thecontent exceeds 70%, the thermal conductivity and elasticity decrease.

The shot content of the silica-alumina ceramic fiber is 1 wt %-10 wt %,preferably 1 wt %-5 wt %, and more preferably 1 wt %-3 wt %. If the shotcontent is less than 1 wt %, manufacture is difficult, and if the shotcontent is 50 wt %, the outer surface of the filter F1 may be damaged.

The fiber length of silica-alumina ceramic fiber is 1 mm-10 mm,preferably 1 mm-50 mm, and more preferably 1 mm-20 mm. If the fiberlength is 1 mm or less, there is a disadvantage in that an elasticstructure cannot be formed. If the fiber length exceeds 100 mm, there isa disadvantage in that the fiber may produce balls of fibers anddecrease the dispersion of inorganic fine particles. Further, if thefiber length exceeds 100 mm, it becomes difficult to make the seal layerthinner than 3 mm and to improve the heat conductance between thefilters F1.

It is preferred that the inorganic binder included in the seal layer 15be a colloidal sol selected from at least one of silica sol and aluminasol. It is especially preferred that silica sol be selected. This isbecause silica sol is optimal for use as an adhesive agent under hightemperatures since it is easily obtained easily sintered to SiO₂. Inaddition, silica sol has a superior insulative characteristic.

In this case, the content of silica sol in the seal layer 15 as a solidis 1 wt %-30 wt %, preferably 1 wt %-15 wt %, and more preferably 5 wt%-9 wt %. If the content is less than 1 wt %, the adhesive strengthdecreases. On the other hand, if the content exceeds 30 wt %, thethermal conductivity decreases.

It is preferred that the organic binder included in the seal layer 15 bea hydrophilic organic high polymer and also be preferred that theorganic binder be a polysaccharide selected from at least one of polyvinyl alcohol, methyl cellulose, ethyl cellulose, and carboxymethylcellulose. It is especially preferred that carboxymethyl cellulose beselected. This is because the seal layer 15 has optimal fluidity due tocarboxymethyl cellulose and thus has superior adhesion under normaltemperatures.

In this case, the content of carboxymethyl cellulose as a solid is 0.1wt %-5.0 wt %, preferably 0.2 wt %-1.0 wt %, and more preferably 0.4 wt%-0.6 wt %. If the content is less than 0.1 wt %, sufficient inhibitionof migration becomes difficult. Migration refers to a phenomenon inwhich the binder in the seal layer 15 moves as the solvent is removed asit dries when the seal layer 15 charged between the sealed bodieshardens. If the content exceeds 5.0 wt %, high temperature burns andeliminates the organic binder and decreases the strength of the seallayer 15.

It is preferred that the inorganic particles included in the seal layer15 be an inorganic powder or an elastic material employing a whiskerthat is selected from at least one of silicon carbide, silicon nitride,and boron nitride. Such carbide and nitrides have an extremely highthermal conductivity and, when included in the surface of a ceramicfiber or in the surface of inside a colloidal sol, contributes toincreasing the thermal conductivity.

Among the above carbide and nitrides, it is especially preferred thatthe silicon carbide powder be selected. This is because the thermalconductivity of silicon carbide is extremely high and easily adapts toceramic fiber. In addition, in the first embodiment, the filter F1,which is the sealed body, is made of sintered porous silicon carbide.Thus, it is preferred that the same type of silicon carbide powder beselected.

In this case, it is preferred that the content of the silicon carbidepowder as a solid be 3 wt %-80 wt %, preferably 10 wt %-60 wt %, andmore particularly, 20 wt %-40 wt %. If the content is 3 wt % or less,the thermal conductivity of the seal layer 15 decreases and results inthe seal layer 15 having a large heat resistance. If the content exceeds80 wt %, the adhesion strength decreases when the temperature is high.

The grain diameter is 0.01 μm-100 μm, preferably 0.1 μm-15 μm, and morepreferably 0.1 μm-10 μm. If the grain diameter exceeds 100 μm, theadhesion and thermal conductivity decrease. If the grain diameter isless than 0.01 μm, the cost of the seal material 15 increases.

The procedure for manufacturing the ceramic filter assembly 9 will nowbe discussed.

First, a ceramic raw material slurry used during an extrusion process, asealing paste used during an end surface sealing process, and a seallayer formation paste used during a filter adhesion process areprepared.

The ceramic raw material slurry is prepared by combining and kneadingpredetermined amounts of an organic binder and water with siliconcarbide particles.

The sealing paste is prepared by combining and kneading an organicbinder, a lubricative agent, a plastic agent, and water with siliconcarbide powder. The seal layer formation paste is prepared by combiningand kneading predetermined amounts of an inorganic fiber, an inorganicbinder, an organic binder, and inorganic particles, and water.

Next, the ceramic raw material slurry is put into an extruder andcontinuously extruded from a mold. Afterward, the extruded honeycombmolded product is cut into equivalent lengths to obtain elongated squarehoneycomb molded product pieces. Further, a predetermined amount ofsealing paste is charged into one of the openings of each cell in thecut pieces such that both end surfaces of each cut piece is sealed.

Then, main sintering is performed by setting predetermined conditions,such as the temperature and time, to completely sinter the honeycombmolded pieces and the sealing bodies 14. All of the sintered poroussilicon carbide filters F1 obtained in this manner are still squarepole-shaped.

The sintering temperature is set to 2,100° C. to 2,300° C. in thepresent embodiment to obtain the average pore diameter of 6 μm-15 μm anda porosity of 35% to 50%. Further, the sintering time is set to 0.1hours to 5 hours. Further, the interior of a furnace has an inertatmosphere during sintering, and the pressure in that atmosphere is thenormal pressure.

Then, after forming a ceramic bedding layer to the outer surface of thefilters F1 as required, the seal layer formation paste is appliedthereto. The outer surfaces of sixteen of such filters F1 are adhered toeach other and thus integrated.

In the following outer form cutting process, the assembly 9, which hasbeen obtained through the filter adherence process and has a squarecross-section, is ground to form the outer shape of the assembly 9 byeliminating unnecessary sections from the peripheral portion of theassembly 9 and form the ceramic filter assembly 9, which cross-sectionis round.

The fine particle trapping performed by the ceramic filter assembly 9will now be described briefly.

The ceramic filter assembly 9 accommodated in the casing 9 a is suppliedwith exhaust gas. The exhaust gas supplied via the first exhaust pipe 6first enters the cells that are opened at the upstream end surface 9 a.The exhaust gas than passes through the cell wall 13 and enters theadjacent cells, or the cells that are opened at the downstream endsurface 9 b. From the openings of these cells, the exhaust gas flows outof the downstream end surfaces 9 b of the filters F1. However, the fineparticles included in the exhaust gas do not pass through the cell walls13 and are trapped by the cell walls 13. As a result, the purified gasis discharged from the downstream end surface 9 b of the filters F1. Thepurified exhaust gas then passes through the second exhaust pipe 7 to beultimately discharged into the atmosphere. The trapped fine particlesare ignited and burned by the catalytic effect that occurs when theinternal temperature of the assembly 9 reaches a predeterminedtemperature.

Example 1-1

(1) 51.5 wt % of a silicon carbide powder having an average graindiameter of 10 μm and 22 wt % of a silicon carbide powder having anaverage grain diameter of 0.5 μm were wet-mixed. Then, 6.5 wt % of theorganic binder (methyl cellulose) and 20 wt % of water were added to theobtained mixture and kneaded. Next, a small amount of the plastic agentand the lubricative agent were added to the kneaded mixture, furtherkneaded, and extruded to obtain the honeycomb molded product. Morespecifically, the a silicon carbide powder having an average particlediameter of about 10 μm was produced by Yakushima Denkou KabushikiKaisha under the product name of C-1000F, and the a silicon carbidepowder having an average particle diameter of about 0.5 μm was producedby Yakushima Denkou Kabushiki Kaisha under the product name of GC-15.

(2) Then, after drying the molded product with a microwave dryer, thethrough holes 12 of the molded product was sealed by the sealing pastemade of sintered porous silicon carbide. Afterward, the sealing pastewas dried again with the dryer. After the end surface sealing process,the dried body was degreased at 400° C. and then sintered for aboutthree hours at 2,200° C. in an argon atmosphere at the normal pressure.This obtained the porous, honeycomb, silicon carbide filters F1.

(3) 23.3 wt % of a ceramic fiber (alumina silicate ceramic fiber, shotcontent 3%, fiber length 0.1 mm-100 mm), 30.2 wt % of silicon carbidehaving an average grain diameter of 0.3 μm, 7 wt % of silica sol (theconverted amount of SiO₂ of the sol being 30%) serving as the inorganicbinder, 0.5 wt % of carboxymethyl cellulose serving as the organicbinder, and 39 wt % of water were mixed and kneaded. The kneadedmaterial was adjusted to an appropriate viscosity to prepare the pasteused to form the seal layer 15.

(4) Then, the seal layer forming paste was uniformly applied to theouter surface of the filters F1. Further, in a state in which the outersurfaces of the filters F1 were adhered to one another, the filters F1were dried and hardened under the condition of 50° C. to 100° C.×1 hour.As a result, the seal layer 15 adhered the filters F1 to one another.The thickness t1 of the seal layer 15 was set at 0.5 mm. The heatconductivity of the seal layer 15 was 0.3 W/mK.

(5) Next, the peripheral portion was cut to shape the peripheral portionand complete the ceramic filter assembly 9, which has a roundcross-section.

Then, the thermal insulative material 10 is wound about the assembly 9obtained in the above manner. In this state, the assembly 9 isaccommodated in the casing 8 and actually supplied with exhaust gas.After a predetermined time elapses, the assembly 9 is removed and cut ata plurality of locations. The cut surfaces were observed with the nakedeye.

Consequently, residuals of the fine particles were not confirmed at theperipheral portion of the assembly 9 (especially, the peripheral portionnear the downstream end surface) where there is a tendency for unburnedparticles to remain. The fine particles were of course completely burnedat other portions. It is considered that such results are obtainedbecause the usage of the seal layer 15 prevents the conductance of heatbetween the filters F1 from being decreased and the temperaturesufficiently increases at the peripheral portion of the assembly 9.Accordingly, in example 1-1, it is apparent that exhaust gas wasefficiently processed.

Examples 1-2, 1-3

In example 1-2, the ceramic filter assembly 9 was prepared by settingthe thickness t1 of the seal layer 15 at 1.0 mm. The other conditionswere basically set in accordance with example 1-1. In example 3, theceramic filter assembly 9 was formed by setting the thickness t1 of theseal layer 15 at 2.5 mm. The other conditions were basically set inaccordance with example 1-1.

Then, the obtained two types of assemblies 9 were used for a certainperiod, and the cut surfaces were observed with the naked eye. The samedesirable results as example 1-1 were obtained. Thus, it is apparentthat the exhaust gas was efficiently processed in examples 1-2 and 1-3.

Example 1-4

In example 1-4, the employed seal layer forming paste was prepared bymixing and kneading 25 wt % of a ceramic fiber (mullite fiber, shotcontent rate 5 wt %, fiber length 0.1 mm-100 mm), 30 wt % of siliconnitride powder having an average grain diameter of 1.0 μm, 7 wt % ofalumina sol (the conversion amount of alumina sol being 20%) serving asan inorganic binder, 0.5 wt % of poly vinyl alcohol serving as anorganic binder, and 37.5 wt % of alcohol. The other portions were formedin accordance with example 1-1 to complete the ceramic filter assembly9. The thickness t1 of the seal layer 15 was set at 1.0 mm. The thermalconductivity of the seal layer 15 was 0.2 W/mK.

Then, the obtained assembly 9 was used for a certain period, and the cutsurfaces were observed with the naked eye. The same desirable results asexample 1 were obtained. Thus, it is apparent that the exhaust gas wasefficiently processed in example 4.

Example 1-5

In example 1-5, the employed seal layer forming paste was prepared bymixing and kneading 23 wt % of a ceramic fiber (alumina fiber, shotcontent rate 4 wt %, fiber length 0.1 mm-100 mm), 35 wt % of boronnitride powder having an average grain diameter of 1 μm, 8 wt % ofalumina sol (the conversion amount of alumina sol being 20%) serving asan inorganic binder, 0.5 wt % of ethyl cellulose serving as an organicbinder, and 35.5 wt % of acetone. The other portions were formed inaccordance with example 1 to complete the ceramic filter assembly 9. Thethickness t1 of the seal layer 15 was set at 1.0 mm. The thermalconductivity of the seal layer 15 was 2 W/mK.

Then, the obtained assembly 9 was used for a certain period, and the cutsurfaces were observed with the naked eye. The same desirable results asexample 1 were obtained. Thus, it is apparent that the exhaust gas wasefficiently processed in example 5.

The ceramic filter assembly 9 of the first embodiment has the followingadvantages:

(1) In each example, the thickness t1 of the seal layer 15 is set in thepreferable range of 0.3 mm-3 mm, and the thermal conductivity of theseal layer 15 is set in the preferable range of 0.1 W/mK-10 W/mK. Thisimproves the thermal conductivity of the seal layer and prevents thethermal conductivity between the filters F1 from being decreased.Accordingly, heat is uniformly and quickly conducted to the entireassembly 9. This prevents a temperature difference from being producedin the assembly 9. Accordingly, the thermal uniformity of the assembly 9is increased and the occurrence of locally unburned particles isavoided. The exhaust gas purification apparatus 1, which uses theassembly 9, has superior exhaust gas processing efficiency.

Further, if the thickness t1 and the thermal conductivity is within theabove range, basic properties, such as adhesiveness and heat resistanceremain the same. This avoids the manufacturing of the seal layer 15 frombecoming difficult. Further, since the seal layer 15 serves to adherethe filters F1 to one another, breakage of the assembly 9 is avoided. Inother words, the assembly 9 is relatively easy to manufacture and hassuperior durability.

(2) The seal layer 15 in each example contains as a solid 10 wt %-70 wt% of ceramic fibers. This enables the seal layer 15 to have high thermalconductivity and elasticity. Thus, the thermal conductivity betweenfilters F1 is improved, and the thermal conductivity of the assembly 9is further increased.

(3) The seal layer 15 in each example contains ceramic fibers, thelengths of which are 100 mm or shorter. Accordingly, the thickness t1 ofthe seal layer 15 may be set to 3 mm or less without any difficulties.This increases the heat conductivity between the filters F1, and thuscontributes to the thermal uniformity of the assembly 9.

(4) The seal layer 15 in each example contains as a solid 3 wt %-80 wt %of inorganic particles. Thus, the seal layer 15 has high thermalconductivity. This increases the heat conductivity between the filtersF1 and contributes to the thermal uniformity of the assembly 9.

(5) The seal layer 15 in the above examples are formed from at least aninorganic fiber, an inorganic binder, an organic binder, and inorganicparticles. Further, the seal layer 15 is made of an elastic materialformed by joining three-dimensionally intersecting the inorganic fiberswith the inorganic particles with an inorganic binder and an organicbinder.

Such material has the advantages described below. Sufficient adhesionstrength is obtained in a low temperature range and a high temperaturerange. Further, the material is elastic. Thus, when thermal stress isapplied to the assembly 9, the release of the thermal stress is ensured.

The first embodiment of the present invention may be modified asdescribed below.

(a) The number of the filters F1 is not limited to 16 and may be anynumber. In this case, filters F1 having different dimensions and shapesmay be combined.

(b) With reference to FIG. 6, in a ceramic filter assembly 21 of afurther embodiment, the filters F1 are offset from one another in adirection perpendicular to the filter axial direction, and the filtersF1 are adhered by the seal layer 15. In this case, the filters F1resists displacement when being accommodated in the casing 8. Thisimproves the breakage strength of the assembly 21. In the ceramic filterassembly 21 of FIG. 6, the seal layer 15 does not include cross-likeportions. It is considered that this contributes to improvement of thebreakage strength. Further, since the thermal conductivity in the radialdirection of the assembly 21 is further improved, the thermal uniformityof the assembly 21 is further enhanced.

(c) Instead of the honeycomb filters F1, the filters may have athree-dimensional mesh structure, a foam-like structure, a noodle-likestructure, or a fiber-like structure.

(d) Prior to the outer form cutting process, the form of the filter F1is not limited to the elongated square shape and may have a triangularpole-like shape or a hexagonal pole-like shape. Further, the assembly 9does not necessarily have to be formed to have a round cross-sectionduring the outer form cutting process and may be formed to have a, forexample, oval cross-section.

FIG. 7 is a perspective view showing a honeycomb filter F10 of a ceramicfilter assembly in a second embodiment of the present invention. FIG. 8is an enlarged cross-sectional view showing the main portion of theexhaust gas purification apparatus. The corners on the outer surface ofthe honeycomb filter F10 are curved to define round surfaces 18. It isrequired that the curvature of the round surfaces 18 be R=0.3 to 2.5. Itis further preferred that the curvature be R=0.7 to 2.5, andparticularly preferred that the curvature be R=1.0 to 2.0.

When the curvature R is 0.3 or less, the corners are still angulated.Thus, the concentration of stress to the corners cannot be sufficientlyavoided and the corners may chip or crack. On the other hand, if thecurvature R exceeds 2.5, the cross-sectional area of the honeycombfilter F1 decreases. This reduces the effective number of cells anddecreases the filtering capability of the assembly 29.

The ceramic filter assembly of the second embodiment is manufactured bychamfering each corner of an elongated square honeycomb molded productpiece and forming the round surfaces 18 with the predetermined curvatureR.

Example 2-1

In example 2-1, the ceramic filter assembly 29 was manufactured bydrying molded products with a microwave dryer, cutting off each cornerto perform chamfering, and forming the round surfaces 18 of R=1.5. Theother steps are in accordance with example 1-1.

An assembly 29 obtained in the above manner was actually supplied withexhaust gas. After a predetermined time, the assembly 29 was removed andobserved with the naked eye.

The result revealed that there were no cracks originating from thecorners in the seal layer 15. Further, there was no chipping of thecorners. Accordingly, it has become apparent that the assembly 29 of theexample 2-1 is extremely superior in strength.

Examples 2-2, 2-3

In example 2, the ceramic filter assembly 9 was manufactured by settingthe curvature of the round surfaces 18 at R=0.4 and forming the otherportions basically in the same manner as in example 2-1. In example 2-3,the ceramic filter assembly 29 was manufactured by setting the curvatureof the round surfaces 18 at R=2.4 and forming the other portionsbasically in the same manner as in example 2-1.

Then, the obtained two types of the assembly 29 were used for a certaintime period in the same manner as example 2-1 and observed with thenaked eye. A preferable result similar to that of example 2-1 wasobtained. In other words, it has become apparent that the assemblies 29of the examples 2-2, 2-3 are superior in strength.

Example 2-4

In example 2-4, the ceramic filter assembly 29 was manufactured by usinga seal layer forming paste in the same manner as in example 1-4 andforming the other portions in the same manner as in example 2-1. Thethickness of the seal layer was set at 1.0 mm, and the curvature of theround surface 18 of each corner was set at R=1.5.

Then, the obtained assembly 29 was used for a certain time period in thesame manner as example 2-1 and observed with the naked eye. A preferableresult similar to that of example 2-1 was obtained. In other words, ithas become apparent that the assembly 29 of example 2-4 is superior instrength.

Example 2-5

In example 2-5, the ceramic filter assembly 29 was manufactured by usinga seal layer forming paste in the same manner as in example 1-5 andforming the other portions in the same manner as in example 2-1. Thethickness of the seal layer was set at 1.0 mm, and the curvature of theround surface 18 of each corner was set at R=1.5.

Then, the obtained assembly 29 was used for a certain time period in thesame manner as example 2-1 and observed with the naked eye. A preferableresult similar to that of example 2-1 was obtained.

Comparative Example

In the comparative example, the ceramic filter assembly 9 wasmanufactured without chamfering the corners and forming the otherportions in the same manner as in example 2-1. Thus, the honeycombfilters F1 of the assembly 29 have angulated corners.

Then, the obtained assembly 29 was used for a certain time period in thesame manner as example 2-1 and observed with the naked eye. Cracks andchipping caused by stress concentration were discovered at multiplelocations. Accordingly, the assembly 29 was inferior in strength.

The ceramic filter assembly of the second embodiment has the advantagesdiscussed below.

(1) The corners on the outer surface of the honeycomb filter F1 areround surfaces 18 having a curvature in an optimal range. This avoidsstress concentration at the corners. Accordingly, the chipping of thecorners of the honeycomb filter F1, the cracking of the seal layer 15from the corners is prevented, and the ceramic filter assembly 29resists breakage. This increases the strength of the assembly 29 andimproves the strength and filtering capability of the exhaust gaspurification apparatus 1, which employs the assembly 29.

(2) The assembly 29 employs the honeycomb filter 1, which is made ofhoneycomb sintered porous silicon carbide. As a result, the obtainedassembly 29 has a higher filtering capability, less pressure loss, andsuperior heat resistance and heat conductance characteristics.

The second embodiment may be modified as described below.

With reference to FIG. 9, the present invention may be embodied in aceramic filter assembly 221 by offsetting the filters F1 from oneanother in a direction perpendicular to the filter axial direction.

Instead of forming the round surfaces 18 by chamfering the corners, theround surfaces may be formed simultaneously when molding the honeycombmolded product with a mold.

The honeycomb filter F1 is not required to be shaped into a rectangularpole, which has a square cross-section, prior to the outer form cuttingprocess. For example, as shown in FIG. 10, a honeycomb filter F20 may beformed into a rectangular pole having a rectangular cross-section.Further, a honeycomb filter F30 may be triangular as shown in FIG. 11,and a honeycomb filter F40 may be hexagonal as shown in FIG. 12.

FIG. 13 is a schematic cross-sectional view showing a ceramic filter 39according to a third embodiment of the present invention.

Referring to FIG. 13 and FIG. 14( b), the ceramic filter assembly 39 ofthe third embodiment has an outer surface 39 c to which a ceramicsmoothing layer 16 is applied. The smoothing layer 16 is made of aceramic material that includes at least ceramic fibers and a binder. Itis preferred that the ceramic material includes inorganic particles,such as silicon carbide, silicon nitride, and boron nitride. It ispreferred that an inorganic binder, such as silica sol or alumina sol,or an organic binder, such as a polysaccharide, be used as the binder.It is preferred that the ceramic material be formed by bindingthree-dimensionally intersecting ceramic fibers and inorganic particleswith a binder. It is preferred that the smoothing layer 16 be formedfrom the same type of material as the seal layer 15, and especiallypreferred that the smoothing layer 16 be made of exactly the samematerial as the seal layer 15.

It is preferred that the thickness of the smoothing layer 16 be 0.1 mmto 10 mm, further preferred that the thickness be 0.3 mm to 2 mm, andoptimal that the thickness be 0.5 mm to 1 mm. If the smoothing layer 16is too thin, pits 17 that are formed in the outer surface 9 c of theceramic filter assembly 9 cannot be completely filled. Thus, gaps tendto remain in such locations. On the other hand, if the smoothing layer16 is thickened, the formation of the layer may become difficult, andthe diameter of the entire assembly 9 may be enlarged.

It is preferred that the seal layer 15 be formed thinner than thesmoothing layer 16, and more particularly, be formed in the range of 0.3mm to 3 mm. When the seal layer 15 is thinner than the smoothing layer,the filtering capacity and the thermal conductance are prevented frombeing decreased beforehand.

The procedure for manufacturing the ceramic filter assembly 39 will nowbe described with reference to FIG. 14.

First, a ceramic raw material slurry used during an extrusion process, asealing paste used during an end surface sealing process, a seal layerformation paste used during a filter adhesion process, and a smoothinglayer formation paste are prepared. When using the seal layer formationpaste to form the smoothing layer, the smoothing layer formation pastedoes not have to be prepared.

The ceramic raw material slurry is prepared by combining and kneadingpredetermined amounts of an organic binder and water with siliconcarbide particles. The sealing paste is prepared by combining andkneading an inorganic binder, a lubricative agent, a plastic agent, andwater with silicon carbide powder. The seal layer formation paste(smoothing layer formation paste) is prepared by combining and kneadingpredetermined amounts of an inorganic fiber, an inorganic binder, anorganic binder, inorganic particles, and water.

Next, the ceramic raw material slurry is put into an extruder andcontinuously extruded from a mold. Afterward, the extruded honeycombmolded product is cut into equivalent lengths to obtain elongated squarehoneycomb molded product pieces. Further, a predetermined amount of thesealing paste is charged into one of the openings of each cell in thecut pieces to seal both end surfaces of each cut piece.

Then, main sintering is performed by setting predetermined conditions,such as the temperature and time, to completely sinter the honeycombmolded pieces and the sealing bodies 14. All of the sintered poroussilicon carbide filters F1 obtained in this manner are still squarepole-shaped.

The sintering temperature is set to 2,100° C. to 2,300° C. in thepresent embodiment to obtain the average pore diameter of 6 μm to 15 μmand a porosity of 35% to 50%. Further, the sintering time is set to 0.1hours to 5 hours. Further, the interior of a furnace has an inertatmosphere during sintering, and the pressure in that atmosphere is thenormal pressure.

Then, after forming a ceramic bedding layer to the outer surface of thefilters F1 as required, the seal layer formation paste is appliedthereto. The outer surfaces of sixteen of such filters F1 are adhered toeach other and thus integrated. At this point, the ceramic filterassembly 39A as a whole has a square cross-section, as shown in FIG. 14(a).

In the following outer form cutting process, the assembly 39A, which hasbeen obtained through the filter adherence process and has a squarecross-section, is ground to form the outer shape of the assembly 9 byeliminating unnecessary sections from the peripheral portion of theassembly 39A.

As a result, the ceramic filter assembly 39 having around cross-sectionis obtained, as shown in FIG. 14( b). Cell walls 13 are partiallyexposed from the surface formed during the outer form cutting. Thus,pits 17 are formed in the outer surface 39 c. The pits 17 are about 0.5mm to 1 mm and are defined by ridges and valleys extending in the axialdirection of the assembly 39 (i.e., the longitudinal direction of thefilters F1).

In the following smoothing layer forming process, the seal layerformation paste is used as the smoothing layer formation paste anduniformly applied to the outer surface 9 c of the assembly 39. Thiscompletes the ceramic filter assembly 39 shown in FIG. 14( c).

Example 3-1

(1) 51.5 wt % of a silicon carbide powder and 22 wt % of β siliconcarbide powder were wet-mixed. Then, 6.5 wt % of the organic binder(methyl cellulose) and 20 wt % of water were added to the obtainedmixture and kneaded. Next, a small amount of the plastic agent and thelubricative agent were added to the kneaded mixture, further kneaded,and extruded to obtain the honeycomb molded product.

(2) Then, after drying the molded product with a microwave dryer, thethrough holes 12 of the molded product were sealed by the sealing pastemade of sintered porous silicon carbide. Afterward, the sealing pastewas dried again with the dryer. After the end surface sealing process,the dried body was degreased at 400° C. and then sintered for aboutthree hours at 2,200° C. in an argon atmosphere at normal pressure. Thisobtained the porous, honeycomb, silicon carbide filters F1.

(3) 23.3 wt % of a ceramic fiber (alumina silicate ceramic fiber, shotcontent 3%, fiber length 0.1 mm-100 mm), 30.2 wt % of silicon carbidehaving an average grain diameter of 0.3 μm, 7 wt % of silica sol (theconverted amount of SiO₂ of the sol being 30%) serving as the inorganicbinder, 0.5 wt % of carboxymethyl cellulose serving as the organicbinder, and 39 wt % of water were mixed and kneaded. The kneadedmaterial was adjusted to an appropriate viscosity to prepare the pasteused to form the seal layer 15 and the smoothing layer 16.

(4) Then, the seal layer forming paste was uniformly applied to theouter surface of the filters F1. Further, in a state in which the outersurfaces of the filters F1 were adhered to one another, the filters F1were dried and hardened under the condition of 50° C. to 100° C.×1 hour.As a result, the seal layer 15 adhered the filters F1 to one another.The thickness t1 of the seal layer 15 was set at 1.0 mm.

(5) Next, the peripheral portion was cut to shape the peripheral portionand complete the ceramic filter assembly 39, which has a roundcross-section. Then, the seal and smoothing paste was uniformly appliedto the expose outer surface 39 c. The smoothing layer 16 having athickness of 0.6 mm was dried and hardened under the condition of 50° C.to 100° C.×1 hour to complete the assembly 39.

The assembly 39 obtained in the above manner was observed with the nakedeye. The pits 17 in the outer surface 39 c were substantially completelyfilled by the smoothing layer 16, and the outer surface 39 c was smooth.Further, there were no cracks in the boundary portions of the smoothinglayer 16 and the seal layer 15. Accordingly, this indicates that thelevels of adhesion and seal were high at the boundary portions.

No gaps were formed in the outer surface 9 c of the assembly 39 whenaccommodating the assembly 39 encompassed by the thermal insulativematerial in the casing 8. Further, when actually supplying exhaust gas,there was no leakage of the exhaust gas through the gaps in the outersurface 39 c from the downstream side. It is thus apparent that exhaustgas is efficiently processed in the third embodiment.

Example 3-2

In example 3-2, the seal and smoothing paste was prepared by mixing andkneading 25 wt % of a ceramic fiber (mullite fiber, shot content rate 5wt %, fiber length 0.1 mm-100 mm), 30 wt % of silicon nitride powderhaving an average grain diameter of 1.0 μm, 7 wt % of alumina sol (theconversion amount of alumina sol being 20%) serving as an inorganicbinder, 0.5 wt % of poly vinyl alcohol serving as an organic binder, and37.5 wt % of alcohol. The other portions were formed in accordance withexample 3-1 to complete the ceramic filter assembly 39.

Then, observations were made by the naked eye in the same manner asexample 1. The pits 17 in the outer surface 39 c were substantiallycompletely filled by the smoothing layer 16. Further, there were nocracks in the boundary portions of the smoothing layer 16 and the seallayer 15. Accordingly, this indicates that the levels of adhesion andseal were high at the boundary portions.

No gaps were formed in the outer surface 39 c of the assembly 39 duringusage. In addition, leakage of exhaust gas through gaps in the outersurface 39 c did not occur. It is thus apparent that exhaust gas wasefficiently processed in example 3-2 in the same manner as example 3-1.

Example 3-3

In example 3-3, the seal and smoothing paste was prepared by mixing andkneading 23 wt % of a ceramic fiber (alumina fiber, shot content rate 4wt %, fiber length 0.1 mm-100 mm), 35 wt % of boron nitride powderhaving an average grain diameter of 1 μm, 8 wt % of alumina sol (theconversion amount of alumina sol being 20%) serving as the inorganicbinder, 0.5 wt % of ethyl cellulose serving as the organic binder, and35.5 wt % of acetone. The other portions were formed in accordance withexample 3-1 to complete the ceramic filter assembly 39.

Then, observations were made by the naked eye in the same manner asexample 3-1. The pits 17 in the outer surface 39 c were substantiallycompletely filled by the smoothing layer 16. Further, there were nocracks in the boundary portions of the smoothing layer 16 and the seallayer 15. Accordingly, this indicates that the levels of adhesion andseal were high at the boundary portions.

No gaps were formed in the outer surface 39 c of the assembly 39 duringusage. In addition, leakage of exhaust gas through gaps in the outersurface 39 c did not occur. It is thus apparent that exhaust gas wasefficiently processed in example 3-3 in the same manner as example 3-1.

Comparative Example

In the comparative example, the smoothing layer 16 was not formed on theouter surface 39 c. The other portions were formed in accordance withexample 3-1 to complete a ceramic filter assembly.

Then, observations were made by the naked eye in the same manner asexample 3-1. There were pits 17 in the outer surface 3-9 c. Thus, gapswere formed in the outer surface 3-9 c during usage of the assembly, andgas leakage through the gaps occurred. Accordingly, in comparison withexamples 3-1 to 3-3, it is apparent that the exhaust gas processingefficiency was inferior.

Accordingly, the ceramic filter assembly 39 has the advantages describedbelow.

(1) The smoothing layer 16 fills the pits 17 and smoothes the outersurface 9 c. Accordingly, gaps are not formed in the outer surface 39 cwhen the assembly 39 is retained. This prevents the leakage of exhaustgas. As a result, the ceramic filter assembly 39 has superior exhaustgas processing efficiency. This, in turn, results in the exhaust gaspurification apparatus 1 having superior exhaust gas processingefficiency.

The smoothing layer 16 is made of ceramic and thus has superior adhesionwith the filter F1, which is made of a sintered porous ceramic, andsuperior heat resistance. Accordingly, even if the assembly 39 isexposed to a high temperature of several hundred degrees Celsius, thesmoothing layer 16 is not burned nor deformed. Thus, the desiredadhesion strength is maintained.

(2) The thickness of the smoothing layer 16 is set in the preferredrange of 0.1 mm to 10 mm. This prevents the leakage of exhaust gaswithout making the manufacture of the assembly 39 difficult.

(3) The seal layer 15 is thinner than the smoothing layer 16. Thisprevents the filtering capability and the thermal conductivity fromdecreasing.

(4) The smoothing layer 16 is made from the same material as the seallayer 15. Since the coefficient of thermal expansion of the smoothinglayer 16 and that of the seal layer 15 are the same, the boundaryportions of the seal and smoothing layer 15, 16 do not crack. In otherwords, high adhesiveness, sealing, and reliability of the boundaryportions are ensured.

Further, a smoothing layer formation paste does not have to be preparedin addition to the seal layer formation paste. This facilitates themanufacture of the assembly 39 and avoids an increase in themanufacturing cost.

(5) The following may be used as the material for forming the seal layer15 and the smoothing layer 16. An elastic material including at least aninorganic fiber, an inorganic binder, an organic binder, and inorganicparticles and bound to one another by the inorganic binder and theorganic binder may be used.

Such material has the advantage described below. The material hassatisfactory adhesion strength in both low temperature and hightemperature ranges. Further, the material is an elastic material. Thus,when thermal stress is applied, the thermal stress is relieved. Further,the material has superior thermal conductance. Thus, heat is uniformlyand quickly conducted to the entire assembly 39. This enables efficientexhaust gas processing.

The third embodiment of the present invention may be modified asdescribed below.

(a) As shown in FIG. 15, the present invention may be embodied in aceramic filter assembly 321 by offsetting the filters F1 from oneanother in a direction perpendicular to the filter axial direction.

(b) The smoothing layer 16 may be formed from a ceramic material thatdiffers from that of the seal layer 15.

(c) The smoothing layer 16 may have the same thickness as the seal layer15 or may have a greater thickness than the seal layer 15.

(d) In addition to forming the smoothing layer 16 by employing anapplication technique, other methods, such as a printing technique, astaining technique, a dip technique, and a curtain coat technique, maybe employed.

FIG. 16 is a schematic perspective view of a ceramic filter assembly 49according to a fourth embodiment of the present invention. The ceramicfilter assembly 49 is formed by a plurality of rectangular pole-likehoneycomb filters F100.

In each honeycomb filter F100, the flow direction of the exhaust gas(direction perpendicular to the filter end surface), which is theprocessed fluid, is defined as the filter length L (mm). Further, thearea obtained when cutting each honeycomb filter F100 in a directionperpendicular to the flow direction (in other words, parallel to thefilter end surface) is defined as the filter cross-sectional area S(mm²).

In this case, the L/S value must be 0.06 mm/mm² to 0.75 mm/mm². It ispreferred that the L/S value be 0.10 mm/mm² to 0.60 mm/mm², and mostpreferred that the L/S value be 0.15 mm/mm² to 0.40 mm/mm².

When the L/S value exceeds 0.75 mm/mm², a temperature difference isproduced in the longitudinal direction of the filter. As a result, ahigh level of thermal stress is applied to the honeycomb filter F100thereby permitting cracks to easily form. On the other hand, when theL/S value is 0.06 mm/mm² or less, a temperature difference is producedin a direction perpendicular to the filter longitudinal direction. Thisalso applies a high level of thermal stress to the honeycomb filter F100thereby permitting cracks to easily form.

It is specifically preferred that the filter length L be 120 mm to 300mm, and especially preferred that the filter length be 140 mm to 200 mm.It is specifically preferred that the filter cross-sectional area S be400 mm² to 2,500 mm², and especially preferred that the cross-sectionalarea S be 600 mm² to 2,000 mm², and especially preferred that thecross-sectional area S be 600 mm² to 2,000 mm². When the values of L andS are outside the preferred range, a temperature difference is producedin the honeycomb filter F100. As a result, a large level of thermalstress easily forms.

Example 4-1

Basically, the same assembly 49 as that of example 1-1 was manufactured.The height W of the filter F100 was 33 mm, the width W2 was 33 mm, andthe length L was 167 mm. Accordingly, the filter cross-sectional area Swas 1,089 mm², and the L/S value was 0.15 mm/mm² (=167/1089).

Then, the thermal insulative material 10 was wrapped around the assembly49. In this state, the assembly was retained in the casing 8 andactually supplied with exhaust gas.

Referring to FIGS. 18(A) and 18(B), thermocouples were embedded in eachof locations P1 to P6 and temperatures T1 to T6 were respectivelymeasured for a certain period, respectively. Further, maximumtemperature differences ΔT(° C.) at each of the locations P1 to P6 wereobtained. The white arrow in the drawing shows the direction of theexhaust gas flow. The temperature measurement was conducted on thehoneycomb filter F100 denoted with reference character X in FIG. 16.

After a predetermined time, the assembly 49 was taken out and thehoneycomb filters F100 were each observed with the naked eye. As aresult, the maximum temperature difference ΔT(° C.) of example 4-1 wasabout 5° C., the value of which is extremely small. Further, cracks werenot confirmed in any of the honeycomb filters F100.

Examples 4-2 to 4-6

In examples 4 to 6, the assembly 49 was manufactured in the same manneras example 4-1. However, in example 4-2, the height W1 of each honeycombfilter F100 was set at 50 mm, the width W2 was set at 50 mm, and thelength L was set at 150 mm. Accordingly, the filter cross-sectional areaS was 2,500 mm², and the L/S value was (150/2,500=) 0.06 mm/mm².

In example 4-3, the height W1 was set at 20 mm, the width W2 was set at20 mm, and the length L was set at 300 mm. Accordingly, the filtercross-sectional area S was 4,000 mm², and the L/S value was (300/400=)0.75 mm/mm².

In example 4-4, the height W1 was set at 33 mm, the width W2 was set at33 mm, and the length L was set at 230 mm. Accordingly, the filtercross-sectional area S was 1,089 mm², and the L/S value was (230/1089)0.21 mm/mm².

In example 4-5 the height W1 was set at 25 m, the width W2 was set at 25m, and the length L was set at 300 mm. Accordingly, the filtercross-sectional area S was 625 mm², and the L/S value was (300/625=)0.48 mm/mm².

In example 4-6 the height W1 was set at 22 m, the width W2 was set at 22m, and the length L was set at 300 mm. Accordingly, the filtercross-sectional area S was 484 mm², and the L/S value was (300/484=)0.62 mm/mm².

An experiment was conducted on the five types of assemblies 59 in thesame manner as in example 4-1. As a result, the maximum temperaturedifference ΔT(° C.) was about 0° C. to 10° C., the values of which areextremely small. Further, no cracks were confirmed in any of thehoneycomb filters F100.

Comparative Example 1

In comparative example 1, the assembly 49 was manufactured in the samemanner as in example 4-1. However, the height W1 of each honeycombfilter F100 was set at 20 mm, the width W2 was set at 20 mm, and thelength L was set at 400 mm. Accordingly, the filter cross-sectional areaS was 1,000 mm², and the L/S value was (400/400=) 1.00 mm/mm².

An experiment was conducted on the assembly 49 in the same manner as inexample 4-1. As a result, the maximum temperature difference ΔT(° C.)was about 30° C. and greater than each embodiment. The length L incomparative example 1 was especially long. Thus, there was a tendency ina temperature difference being produced in the longitudinal direction ofthe filter.

Further, cracks were confirmed in some of the honeycomb filters F100,and the honeycomb filters F100 were damaged.

Comparative Example 2

In comparative example 2, the assembly 49 was manufactured in the samemanner as in example 4-1. However, the height W1 was set at 70 mm, thewidth W2 was set at 70 mm, and the length L was set at 167 mm.Accordingly, the filter cross-sectional area S was 4,900 mm², and theL/S value was (167/4,900=) 0.03 mm/mm².

An experiment was conducted on the assembly 49 in the same manner as inexample 4-1. As a result, the maximum temperature difference ΔT(° C.)was about 20° C. and greater than each embodiment. The filtercross-sectional area S in comparative example 2 was especially large.Thus, there was a tendency in a temperature difference being produced ina direction perpendicular to the longitudinal direction of the filter.Further, cracks were confirmed in some of the honeycomb filters F100,and the honeycomb filters F100 were damaged.

The advantages of the ceramic filter assembly 49 of the fourthembodiment will be discussed below.

(1) By setting the ratio L/S between the filter length L and the filtercross-sectional area in the preferable range, the production of a largethermal stress is prevented without producing a large temperaturedifference in each of the honeycomb filters F100. This prevents cracksfrom being produced in the honeycomb filters F100 and the honeycombfilters F100 resist being damaged. Due to the increase in the strengthof each honeycomb filter F100, the ceramic filter assembly 49 ismanufactured with superior strength. Further, the employment of theassembly 49 increases the strength of the exhaust gas purificationapparatus 1 and enables usage over a long period.

The fourth embodiment may be modified as described below.

(a) As long as the condition of the L/S value being in the range of 0.06mm/mm² to 0.75 mm/mm² is satisfied, the form of the honeycomb filterF100 may be changed to a cylindrical pole-like shape, a triangularpole-like shape, or a hexagonal pole-like shape.

(b) In addition to using the honeycomb filters F100 as a member formingthe ceramic filter 49, the honeycomb filter F100 may be used as anindependent filter.

FIG. 19 is a perspective view showing a honeycomb filter 59 having ahoneycomb structure according to a fifth embodiment of the presentinvention. FIG. 20 is a cross-sectional view taken along line 20-20 ofthe filter 59 of FIG. 19. FIG. 21 is a cross-sectional view showing amain portion of an exhaust gas purification apparatus.

It is preferred that the cell density of the honeycomb filter 59 be120/inch (18/cm²) or greater, and more specifically, be in the range of120 to 180/inch. When the cell density is less than 120, the area ofcontact with the exhaust gas decreases. This lowers the purificationcapability of the honeycomb filter 9.

It is preferred that the thickness of the cell wall 13 be 0.46 mm orless, and more specifically be in the range of 0.20 to 0.46 mm. When thethickness of the cell wall 13 exceeds 0.46 mm, the opening area of thecell decreases and the area of contact with the exhaust gas decreases.This lowers the purification capability of the honeycomb filter 9.Further, if the cell wall 13 is made thicker than 0.46 mm whilemaintaining the cell opening area, the entire honeycomb filter 9 isenlarged.

It is preferred that the average pore diameter of the honeycomb filter 9be 5 μm to 15 μm, and further preferred that the average pore diameterbe 8 μm to 12 μm. If the average pore diameter is less than 5 μm, thedeposit of particulates clogs the honeycomb filter 9. This increasespressure loss. Thus, the driving performance of the vehicle falls, fuelefficiency decreases, and the driving feel becomes unsatisfactory. Onthe other hand, if the average pore diameter exceeds 50 μm, fineparticles cannot be trapped. This decreases the trapping efficiency anddeteriorates the particulate filtering function.

It is preferred that the porosity of the honeycomb filter 9 be 30% to50%, and further preferred that the porosity be 35% to 49%. If theporosity is less than 30%, the honeycomb filter 9 becomes too dense.This hinders the interior flow of exhaust gas. If the porosity exceeds50%, the number of pores in the honeycomb filter 9 becomes excessive.This may decrease the strength and lower the trapping efficiency of fineparticles.

Among the pores of the honeycomb filter 9, it is preferred that 20% ormore be through pores. More specifically, it is preferred that 20% to80% be through pores, and especially preferred that 20% to 50% bethrough pores. A through pore refers to a gap that extends through acell wall 13 and connects adjacent holes 12. If the through pores areless than 20% of the pores, the pressure loss becomes large. Thus, thedriving performance of the vehicle falls, fuel efficiency decreases, andthe driving feel becomes unsatisfactory. On the other hand, if thethrough pores exceed 80% of the pores, manufacture may become difficultand cause stable material supply to be difficult.

It is preferred that total volume of the honeycomb filter 9 be ¼ to 2times the total displacement of the engine. It is further preferred thatthe total volume be ½ to 1.5 times the total displacement. If the valueis less than ¼, the deposit of particulates clogs the honeycomb filter9. If the value exceeds 2 times, the honeycomb filter 9 is enlarged.When the honeycomb filter 9 is enlarged, there is a tendency of thetemperature differing between portions of the filter 9 duringcombustion. This increases the thermal stress applied to the honeycombfilter 9 and increases the possibility of the formation of cracks.

The honeycomb filter 9 is made of sintered porous silicon carbide, whichis a type of sintered carbide. The impurities included in the sinteredporous silicon carbide is 5 wt % or less. It is preferred that theamount of impurities be 1 wt % or less and it is especially preferredthat the amount of impurities be 0.1 wt % or less. If the impuritiesexceed 5 wt %, impurities concentrate at the grain boundary of thesilicon carbide crystal grains and significantly decreases the strengthat the grain boundary (strength bonding crystal grains). This makes thegrain boundary vulnerable to breakage. Impurities include Al, Fe, O andfree C. Like the honeycomb filter 9, the honeycomb filter 9 is made ofsintered porous silicon carbide.

Example 5-1

Basically, in the same manner as the example 4-1, the through holes 12of the molded product were dried with a microwave dryer and then sealedwith a sealing paste made of sintered porous silicon carbide. Afterward,the drier was used again to dry the sealing paste. Subsequent to the endsealing process, the dried product was degreased at 400° C. and thensintered for about three hours at 2,250° C. in an argon atmosphere undernormal pressure.

As a result, the produced sintered porous silicon carbide honeycombfilter 59 had a pore diameter of 10 μm, a porosity of 42%, a throughpore existence rate of 25% relative to the pores, a cell density of150/inch, and a cell wall 13 thickness of 0.4 mm. The honeycomb filter59 had a diameter of 100 mm, a length of 200 mm, and a total volume of2,300 cm³. The total volume refers to the volume obtained by subtractingthe volume of the through holes 12 from the volume of the entirehoneycomb filter 59. It is preferred that the thickness of the cell wall13 be 0.46 mm or less, and more specifically, in the range of 0.20 to0.46 mm.

Then, the honeycomb filter 59 was wrapped around the honeycomb filter59. In this state, the honeycomb filter 59 was retained in the casing.An engine having a displacement of about 3,000 cc was then used tosupply the exhaust gas purification apparatus 1 with exhaust gas at aflow rate of 7 m/sec. In this state, the pressure value of the exhaustgas at the upstream side of the honeycomb filter 59 and the pressurevalue of the exhaust gas at the downstream side were measured. Apressure loss ΔP (mmAq), which is the difference between the values, wasobtained. Further, the amount of soot at the rear side of the honeycombfilter 59 was measured to confirm the amount of particulates that werenot trapped. Further, a certain time period, the honeycomb filter 59 wastaken out and observed with the naked eye to confirm cracks. The resultsare shown in table 1.

TABLE 1 Soot Average Existence Amount Pore Average Rate of PressureBehind Flexural Total Filter Type of Diameter Porosity Through Loss ΔPFilter Strength Volume Ceramic (μm) (%) Pores (%) (mmAq) (g/km) (Mpa)(cm³) Cracks Example 1 Silicon 10 42 25 80 0.01 6.5 2300 None CarbideExample 2 Silicon 6 38 30 100 0.01 6.2 2300 None Carbide Example 3Silicon 14 48 45 60 0.015 6.0 2300 None Carbide Comparative Silicon 3 1010 300 0.005 7.2 700 None Example 1 Carbide Comparative Silicon 20 70 1540 0.04 2.5 7000 Confirmed Example 2 Carbide Comparative Cordierite 3020 15 120 0.015 3.1 700 Confirmed Example 3

As shown in table 1, the pressure loss ΔP in example 5-1 was about 80mmAq, the value of which is extremely small. The particulate leakageamount was about 0.01 g/km, the value of which is extremely small. Thehoneycomb filter 9 had a flexural strength of 6.5 Mpa and had anextremely high level of mechanical strength. There were no cracks in thehoneycomb filter 9.

Example 5-2, 5-3

In examples 5-2 and 5-3, the honeycomb filter 59 was manufacturedbasically in the same manner as in example 5-1. However, in examples 5-2and 5-3, only the total volume of the honeycomb filter 59 was the sameas that of example 5-1. The mixture ratio, sintering temperature,sintering time, etc. were changed as described below to adjust the porediameter, porosity, and the through pore existence rate relative to thepores.

In the produced sintered porous silicon carbide honeycomb filter 59 ofexample 5-2, the pore diameter was 6 μm, the porosity was 32%, and thethrough pore existence rate was 30%. The same experiment as that ofexample 5-1 was conducted. The pressure loss ΔP was about 100 mmAq, thevalue of which is extremely small. The particulate leakage amount wasabout 0.01 g/km, the value of which is extremely small. The honeycombfilter 59 had a flexural strength of 6.2 Mpa and had an extremely highlevel of mechanical strength. Further, there were no cracks in thehoneycomb filter 59.

In the produced sintered porous silicon carbide honeycomb filter 59 ofexample 5-3, the pore diameter was 14 μm, the porosity was 48%, and thethrough pore existence rate was 45%. In the experiment result of thisexample, the pressure loss ΔP was about 60 mmAq, the value of which isextremely small. The particulate leakage amount was about 0.015 g/km,the value of which is extremely small. The honeycomb filter 59 had aflexural strength of 6.0 Mpa and had an extremely high level ofmechanical strength. There were no cracks in the honeycomb filter 59.

Comparative Examples 1 to 3

In comparative examples 1 to 3, honeycomb filters were manufacturedbasically in the same manner as in example 5-1. However, in comparativeexample 1, the total volume of the honeycomb filter was 700 cm³, whichis less than ¼ the displacement (3,000 cc). Further, the pore diameter,porosity, and the through pore existence rate relative to the pores wasas described below.

In the produced sintered porous silicon carbide honeycomb filter ofcomparative example 1, the pore diameter was 3 μm, the porosity was 10%,and the through pore existence rate was 10%. In the experiment result ofcomparative example 1, the pressure loss ΔP was about 300 mmAq, thevalue of which is extremely large. The particulate leakage amount wasabout 0.005 g/km, the value of which is extremely small. The honeycombfilter had a flexural strength of 7.2 Mpa and had an extremely highlevel of mechanical strength. There were no cracks in the honeycombfilter.

In comparative example 2, the total volume of the honeycomb filter wasgreater than that of examples 1-3 and was 7,000 cm³, which is two timesor greater than the displacement (3,000 cc). Further, in the producedsintered porous silicon carbide honeycomb filter, the pore diameter was20 μm, the porosity was 70%, and the through pore existence rate was15%. In the experiment result of comparative example 2, the pressureloss ΔP was about 40 mmAq, the value of which is extremely small. Theparticulate leakage amount was about 0.04 g/km, the value of which isextremely small. The honeycomb filter had a flexural strength of 2.5 Mpaand satisfactory mechanical strength was not obtained. There were nocracks in the honeycomb filter.

In comparative example 3, a cordierite honeycomb filter was producedthrough a known manufacturing method that differs from the manufacturingmethod of comparative examples 1 and 2. The total volume of thehoneycomb filter was 700 cm³. In the honeycomb filter, the pore diameterwas 30 μm, the porosity was 20%, and the through pore existence rate was15%. In the experiment result of comparative example 3, the pressureloss ΔP was about 120 mmAq, the value of which is large. The particulateleakage amount was about 0.015 g/km, the value of which is large. Thehoneycomb filter had a flexural strength of 3.1 Mpa and satisfactorymechanical strength was not obtained. There were no cracks in thehoneycomb filter.

Table 1 shows the comparison result of examples 5-1 to 5-3 andcomparative examples 1 to 3, as described above.

(Experiment Result)

As apparent from table 1, it was confirmed that exhaust gas passedsmoothly through all of the honeycomb filters 59 in examples 5-1 to 5-3.Further, the particulate leakage amount was substantially null, and therequired mechanical strength of the honeycomb filter was obtained. Incomparison, the required mechanical strength of the honeycomb filter wasobtained in comparative example 1. However, exhaust gas did not passsmoothly through the honeycomb filter. Further, in comparison example 2,exhaust gas passed smoothly through the honeycomb filter. However, therequired mechanical strength was not obtained. In example 3, exhaust gasdid not pass smoothly through the honeycomb filter, and the requiredmechanical strength was not obtained.

The advantages of the honeycomb filter 59 of the fifth embodiment willnow be discussed.

(1) The sintered porous silicon carbide honeycomb filter 59 is arrangedin the casing 8. The honeycomb filter 9 is formed so that the averagepore diameter is 5 to 15 μm, the average porosity is 30 to 40%, and thethrough pore existence rate relative to the pores is 20% or greater.Since the honeycomb filter 9 is not excessively dense, exhaust gaspasses smoothly through the interior, and pressure loss is decreased.This improves fuel efficiency and prevents deterioration of the drivingfeel. Further, since the amount of gaps in the honeycomb filter 9 is notexcessive, fine particulates are trapped and the trapping efficiency isimproved. Additionally, even if the honeycomb filter 9 is porous,satisfactory mechanical strength is obtained. Thus, the producedhoneycomb filter 9 resists breakage caused by vibrations and thermalimpact.

(2) The honeycomb filter 9 is formed so that the average pore diameteris 8 to 12 μm, the average porosity is 35 to 49%, and the through poreexistence rate relative to the pores is 20 to 50% or greater. Thus, thepressure loss is further decreased, and the strength is increased.

(3) The end surfaces of the honeycomb filter 9 so that the sealingbodies 14 seal the cells alternately. The number of cells per squareinch is 120 or more, and the thickness of the cell wall 13 is 0.46 mm orless. This increases the area of contact with the exhaust gas andincreases the purification capability of the honeycomb filter 9.

(4) The total volume of the honeycomb filter 9 is ¼ to 2 times the totaldisplacement of the diesel engine 2. Since the deposit amount of theparticulates does not become excessive, clogging of the honeycomb filter9 is prevented. Further, the honeycomb filter 9 is not enlarged. Thisprevents the occurrence of temperature differences between differentlocations of the honeycomb filter 9 during combustion. Accordingly, thethermal stress applied to the honeycomb filter 9 is decreased and cracksare not produced.

The fifth embodiment may be modified as described below.

(a) The form of the honeycomb filter 9 is not limited to a cylindricalpole-like shape and may be changed to a cylindrical pole-like shape, atriangular pole-like shape, or a hexagonal pole-like shape.

(b) As shown in FIG. 22, a plurality (16) of honeycomb filters 523 maybe integrated to manufacture a ceramic filter assembly 521. In eachpolygonal honeycomb filter 523, the average pore diameter is 8 to 12 μm,the average porosity is 35 to 49%, and 20 to 50% of the pores arethrough pores. The outer surfaces of the honeycomb filters 523 areconnected to one another by a ceramic seal layer 522.

In a sixth embodiment, a specific surface area of the particles formingthe cell wall 13 of the honeycomb filter 59 is 0.1 m²/g or more, andmore specifically, 0.1 to 1 m²/g. If the specific surface area of thecell walls 13 is 0.1 m²/g or less, the deposit of the particulates clogsthe honeycomb filter 59. This increases pressure loss and thus decreasesthe fuel efficiency of the vehicle and degrades the feeling drive. Ifthe specific surface area exceeds 1.0 m²/g, fine particulates cannot betrapped. This decreases the trapping efficiency and causes the filteringfunction of the honeycomb filter 59 to become unsatisfactory.

Example 6-1

A honeycomb filter 59 was produced basically in the same manner as thatof example 5-1 and the specific surface area of the particles formingthe cell wall 13 was 0.3 m²/g. In example 6-2 and the comparativeexample, honeycomb filters 59 were produced basically in the same manneras example 5-1. The specific surface area of the honeycomb filter 59 ofexample 6-2 was 0.8 m²/g, and the specific surface area of the honeycombfilter 59 of the comparative example was 0.05 m²/g. In each of thehoneycomb filters 50 of examples 6-1, 6-2 and the comparative example,the cell density was 150/inch and the thickness of the cell wall 13 was0.4 mm.

The honeycomb filter 59 was wrapped by the thermal insulative material10. In this state, the honeycomb filter 59 was retained in the casing 8.A diesel engine 2 having a displacement of about 3,000 cc was then usedto supply the exhaust gas purification apparatus 1 with exhaust gas at aflow rate of 9 m/sec. In this state, the pressure value of the exhaustgas at the upstream side of the honeycomb filter 59 and the pressurevalue of the exhaust gas at the downstream side were measured. Apressure loss ΔP (mmAq), which is the difference between the values, wasobtained. The results are shown in table 2.

TABLE 2 Comparative Example 1 Example 2 Example Specific Surface 0.3 0.80.05 Area (cm²/g) Particulate 180 120 250 Pressure Loss (mmAq)

As apparent from table 2, the pressure loss ΔP of the honeycomb filters59 in example 6-1, example 6-2, and the comparative example was 180mmAq, 120 mmAq, and 250 mmAq, respectively. Accordingly, in examples 6-1and 6-2, a large pressure loss such as that of the comparative examplewas not confirmed.

The honeycomb filter 59 of the sixth embodiment has the advantagesdescribed below.

(1) In the honeycomb filter 9, the specific surface area of theparticles forming the cells wall 13 is 0.1 m²/g or greater. Since thehoneycomb filter 9 does not become excessively dense, exhaust gas passessmoothly though the interior, and the pressure loss is decreased.Accordingly, fuel efficiency is improved and degradation of the drivingfeel is prevented. In addition, the upper limit of the specific surfacearea of the particles is 1.0 m²/g. Thus, the gap amount of the honeycombfilter 9 is not excessive and the trapping of fine particles is ensured.This improves the trapping efficiency.

(2) The sintered silicon carbide cell wall 13 has superior heatresistance. This prevents the cell wall 13 from being deformed or burnedaway. Accordingly, fluid is efficiently purified over a long timeperiod.

(3) The porous cell wall 13 enables smooth passage of the exhaust gasand further decreases power loss. In addition, the trapping efficiencyof particulates is further increased.

The sixth embodiment may be modified as described below.

A plurality (16) of honeycomb filters may be integrated to manufacture aceramic filter assembly. The specific surface area of the cell wall ofeach honeycomb filter is 0.1 to 1 m²/g.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

1. A honeycomb filter comprising: an outer wall having a substantiallycylindrical shape and a central axis of the substantially cylindricalshape; first walls provided at an inner side of the outer wall andextending along the central axis; second walls provided at the innerside of the outer wall and extending along the central axis, the firstwalls and the second walls substantially perpendicularly crossing todefine cells having a substantially square shape in a cross-sectionperpendicular to the central axis; and a thickness of the outer wall inthe cross-section at a first intersection between the outer wall and afirst diameter substantially parallel to the first walls in thecross-section and a thickness of the outer wall in the cross-section ata second intersection between the outer wall and a second diametersubstantially parallel to the second walls in the cross-section beinglargest.
 2. The honeycomb filter according to claim 1, wherein, in thecross-section, end cells formed between the outer wall and an outermostfirst wall among the first walls have a substantially square cell at thefirst intersection and a trapezoidal cell at a position away from thefirst intersection along the outermost first wall.
 3. A honeycomb filtercomprising: cell walls extending along a longitudinal direction of thehoneycomb filter to define cells between the cell walls; and the cellshaving first cells and second cells each having a substantially squareshape in a cross-section perpendicular to the longitudinal direction,the first cells being sealed and the second cells being open at one endface of the honeycomb filter, the substantially square shape of thefirst cells including a first side whose extended line intersects asecond side of the substantially square shape of the second cells, thesecond side and the extended line of the first side being substantiallyperpendicular.
 4. A method for producing a honeycomb filter, comprising:providing a ceramic raw material slurry; providing the ceramic rawmaterial slurry into an extruder to obtain a molded honeycomb filter,the molded honeycomb filter comprising: an outer wall having asubstantially cylindrical shape and a central axis of the substantiallycylindrical shape; first walls provided at an inner side of the outerwall and extending along the central axis; second walls provided at theinner side of the outer wall and extending along the central axis, thefirst walls and the second walls substantially perpendicularly crossingto define cells having a substantially square shape in a cross-sectionperpendicular to the central axis; and a thickness of the outer wall inthe cross-section at a first intersection between the outer wall and afirst diameter substantially parallel to the first walls in thecross-section and a thickness of the outer wall in the cross-section ata second intersection between the outer wall and a second diametersubstantially parallel to the second walls in the cross-section beinglargest; and sintering the molded honeycomb filter.
 5. A method forproducing a honeycomb filter, comprising: mixing ceramic particles and abinder to obtain a ceramic raw material slurry; providing the ceramicraw material slurry into an extruder to obtain a molded honeycombfilter, the molded honeycomb filter comprising: cell walls extendingalong a longitudinal direction of the molded honeycomb filter to definecells having a substantially square shape; drying the molded honeycombfilter using a microwave dryer; and sintering the dried molded honeycombfilter.
 6. The honeycomb filter according to claim 1, wherein theassembly is formed from a sintered porous silicon-carbide.
 7. Thehoneycomb filter according to claim 1, wherein an outer surface of eachcell wall carries at least one oxide catalyst selected from a platinumgroup element, other metal elements and oxides of these metal elements.8. A method for producing a honeycomb filter, comprising: providing aceramic raw material slurry; providing the ceramic raw material slurryinto an extruder to obtain a molded honeycomb filter, the moldedhoneycomb filter comprising: cell walls extending along a longitudinaldirection of the honeycomb filter to define cells between the cellwalls; and the cells having first cells and second cells each having asubstantially square shape in a cross-section perpendicular to thelongitudinal direction, the first cells being sealed and the secondcells being open at one end face of the honeycomb filter, thesubstantially square shape of the first cells including a first sidewhose extended line intersects a second side of the substantially squareshape of the second cells, the second side and the extended line of thefirst side being substantially perpendicular; and sintering the moldedhoneycomb filter.